1. Field of the Invention
The present invention relates to arrangements for detecting islanding conditions in an electrical power distribution system.
2. Background Art
In electric utility systems, an open circuit condition can cause the creation of an xe2x80x9cislandxe2x80x9d of electrical load and generation. These conditions are undesirable and of a particular concern in distributed power generation systems having generation and loads coexisting on a distribution feeder. The island can result in an abnormal voltage or frequency being supplied to the load.
Islanding detection arrangements typically operate by detecting under/over-voltage conditions and/or under/over-frequency conditions in a distribution grid. Typical arrangements either detect whether a generator speed has increased or decreased due to power imbalance, or detect whether the generator voltage is too high or low as a result of either the real or reactive power of the generator not matching the loads placed thereon.
Most detection arrangements, like the foregoing, are generally limited to those conditions in which a distributed power generator is unable to support a load.
Moreover, these arrangements tend to be effective only when the load is significantly different than the generator power. Accordingly, when a load is well matched to the generator""s output, most detection arrangements tend to experience difficulties in detecting an islanded load. As such, a need exists for an improved islanding detection arrangement that can detect open circuit conditions even when a load is well matched to a generator power output.
Accordingly, the present invention provides an anti-islanding arrangement for detecting an impedance of a power grid. The arrangement provides an indication of changes in the grid impedance that are indicative of islanding or open-phase conditions.
In accordance with one aspect of the present invention, a signal is injected into the grid so that the resulting impedance response can be measured. If the magnitude or phase angle of the measured impedance on any phase exceeds a predetermined threshold or deviates from a desired range, an open phase or islanding condition can be indicated.
In accordance with another aspect of the present invention, an arrangement is provided for detecting an islanding condition in an electrical power supply system that includes a central power generator for providing electrical power, at least one distributed power generator for operating as a local source of electrical power for a localized load, and a feeder network coupled to the central power generator and the at least one distributed power generator for providing electrical power to the localized load. A signal injector is connected to the power supply system for injecting a signal into each phase of the generated power so that an impedance calculator connected to the power supply system can determine the resulting system impedance. A controller then can identify the occurrence of an islanding condition based on a determination made by a comparator arranged to compare the determined system impedance with a predetermined threshold.
Thus, the present invention advantageously allows detection of islanding conditions when the load is well matched to the generator power.